1. Field of the Invention
The present invention relates to a technique of, in an electroencephalogram measurement system which determines a state or intent of a user from a characteristic signal of the electroencephalogram of the user and feeds back the result of determination, determining a state of attachment of the electrodes which are utilized for electroencephalogram measurement.
2. Description of the Related Art
Conventionally, medical institutions have utilized electroencephalograms for diagnosis of epilepsy, Alzheimer's disease, and the like. On the other hand, there have been attempts at developing an electroencephalogram interface for inferring the psychological state of a healthy user based on his or her electroencephalogram and inferring his or her intent of manipulation or intent of selection with respect to a device.
For example, as an electroencephalogram interface for use with a healthy user, Japanese Laid-Open Patent Publication No. 7-108848 discloses an “apparatus for preventing drowsy driving” which infers the state of a user. The electroencephalogram interface of Japanese Laid-Open Patent Publication No. 7-108848 subjects an electroencephalogram to a frequency analysis and determines the user's drowsiness from the frequency power of an a wave component (8 Hz to 13 Hz)(see paragraph [0010]). In “The Mental Prosthesis Assessing the Speed of a P300-Based Brain-Computer Interface”, TRANSACTIONS ON REHABILITATION ENGINEERING 2000, Vol. 8, June 2000 (herein referred to as “Non-Patent Document”, Emanuel Donchin et al. also propose an interface for inferring an intent of selection of a user. Among event-related potentials of the electroencephalogram, the interface of the Non-Patent Document uses a characteristic signal called P300 to distinguish an option that a user wishes to select.
In the traditional electroencephalogram measurements mentioned above, a highly electrically conductive cream, called “paste”, is applied on the electrodes, and someone else (not the user) needs to ensure that the electrodes are in tight contact with the skin (scalp) of the user.
Such manners of use, which require use of paste or help of someone else for wearing ease, will present burdens to the user an electroencephalogram interface is to be used in a daily-life environment. Now, an example will be discussed where an electroencephalogram measurement apparatus is incorporated in a wearable device such as a head-mount display (Head-Mount-Display: HMD) to measure an electroencephalogram for an electroencephalogram interface. Upon using an HMD, a user needs to be able to wear the electroencephalogram measurement apparatus together with the HMD at the same time. Moreover, the act of applying paste onto the electrodes also presents a burden on the user. Furthermore, after the device is detached, the paste will remain where the electrodes have been placed, and thus needs to be wiped off by the user himself or herself.
Therefore, in order for the user to easily wear an electroencephalogram measurement device by himself or herself, it is preferable to adopt electrodes which do not require use of paste (hereinafter referred to as “dry electrodes”).
However, use of dry electrodes presents a problem in wearing stability. As one example, when a force acts on a dry electrode, the state of contact between the skin and the dry electrode will change because there is no paste. As a result, problems may occur, such as the electrode position changing even though contact with the skin may be maintained (“electrode shifting”), or a space being created between the skin and the electrode to disable electroencephalogram measurement (“lifting”, “disengagement”). Note that a paste has a high viscosity, and serves not only to enhance the electrical conductivity between the skin and the electrode but also to prevent electrode shifting and electrode disengagement. This produces an effect of conserving the state of contact between the skin and the electrode even when the position of an electrode slightly is changed due to a force acting thereupon, because the paste with a high viscosity will then be deformed.
When electrode shifting occurs, the skin will be rubbing against the electrode surface, so that noises may likely be mixed. If an electrode disengagement occurs, electroencephalogram measurement will be so affected that it may not be continued. Since the user will not always be in a resting state but will undergo various motions in a daily-life environment, insufficiencies concerning electrode contact, such as electrode shifting and electrode disengagement, are probably likely to occur.
Therefore, when any insufficiency such as electrode shifting or electrode disengagement occurs, it is necessary to quickly detect that a situation obstructing electroencephalogram measurement has occurred.
Conventionally known methods for detecting electroencephalogram measurement insufficiencies are the methods in Japanese Examined Utility Model Publication No. 7-3347, Japanese Laid-Open Patent Publication No. 2006-212348, and Japanese Laid-Open Patent Publication No. 2006-6665 described below.
In Japanese Examined Utility Model Publication No. 7-3347, a weak current is allowed to flow through an electroencephalogram electrode, and a resistance value (contact resistance) between the skin and the electrode is calculated from the measured voltage value. As a result, the state of contact between the skin and the electrode is estimated, insufficiencies concerning the state of electrode attachment can be detected (see page 3, left column, second paragraph).
In Japanese Laid-Open Patent Publication No. 2006-212348, FIG. 2 shows that a coil is provided near an electrode, and that a voltage is applied to the coil. Based on whether a resultant induced current in the electrode is superposed on the electroencephalogram waveform or not, it is possible to determine whether the electrode and the scalp are in contact (paragraph [0038]).
In Japanese Laid-Open Patent Publication No. 2006-6665, measurements are taken of a plurality of “electroencephalogram channels”, each electroencephalogram channel defining an electroencephalogram signal to be measured based on the potential difference between a pair of electrodes. In other words, a plurality of pairs of electrodes are provided, and an electroencephalogram signal is measured by each pair. Then, for each electroencephalogram channel, S (i.e., the signal to be measured) and N (i.e., any signal other than the signal to be measured) are calculated. Through a comparison of the S/N ratio against a threshold value, it is determined as to which electroencephalogram channel is suffering from a measurement insufficiency (paragraph [0028]).
Japanese Laid-Open Patent Publication No. 2006-14833 discloses cerebral function judging system 1. The cerebral function judging system 1 judges the contact state of each electrode 2 of the brain wave signal primary detecting element 3. After having judged that a contact state is fine, the system conducts cerebral function analysis. Thus, a cerebral function state can be judged with sufficient accuracy. Since the system does not start inspections before having judged that a contact state is fine, the system can prevent from reducing analysis accuracy of cerebral function state or spoiling inspection results before the system starts the inspection (paragraph [0020]).
However, any of the techniques of the aforementioned conventional approaches has a problem of being unable to identify which electrode is suffering from an insufficiency in the electroencephalogram measurements in a daily-life environment. The respective problems of the above techniques will be described in detail below.
Firstly, in the approach of Japanese Examined Utility Model Publication No. 7-3347, it is necessary to stop the electroencephalogram measurement before measuring the contact resistance. However, this approach is based on the premise that the state of contact of an electrode will not change very much, such that once a contact resistance is measured at the beginning of measurement, there is no need to take more measurements. Accordingly, no thoughts are given to the reconciliation between electroencephalogram measurements and contact resistance measurements. However, the state of electrode attachment will change over time in a daily-life environment, and thus the frequency with which to confirm the state of attachment should increase, but stopping the electroencephalogram measurement each time before a contact resistance measurement will be considerably inconvenient.
In the approach of Japanese Laid-Open Patent Publication No. 2006-212348 of determining whether an electrode is in contact with the skin or not, no consideration is given to the insufficiencies concerning the state of contact of the electrode, e.g., shifting of the electrode that may occur in response to the user's motion, or changes in the pressure from the electrode.
Thus, in Japanese Examined Utility Model Publication No. 7-3347, and Japanese Laid-Open Patent Publication No. 2006-212348, no consideration is given to the detection of insufficient electrodes in an environment where the state of electrode attachment may change, making it difficult to identify which electrode has become insufficient.
On the other hand, in Japanese Laid-Open Patent Publication No. 2006-6665, insufficiencies of electroencephalogram measurement can also be detected even in an environment where the state of attachment may change. However, it is still impossible to identify which electrode is suffering from insufficient wearing.
Note that, even by using paste, electrode shifting and electrode disengagement cannot be completely eliminated. Therefore, the above-described problems of the conventional techniques are not confined to dry electrodes alone. Also in the case where electrode shifting or electrode disengagement occurs in an electrode where paste is applied, it is still necessary to identify which electrode has become insufficient.